1. Field of the Invention
This invention relates to a radiation image read-out and reproducing method for obtaining an image signal by reading out a radiation image from a recording medium on which the radiation image has been recorded, and reproducing a visible image by use of the image signal, and an apparatus for carrying out the method. This invention particularly relates to a radiation image read-out and reproducing method which is suitable for ascertaining a change in density or the like between radiation images of the same object.
2. Description of the Prior Art
Techniques for reading out a recorded radiation image in order to obtain an image signal, carrying out appropriate image processing on the image signal, and then reproducing a visible image by use of the processed image signal have heretofore been known in various fields. For example, as disclosed in Japanese Patent Publication No. 61(1986)-5193, an X-ray image is recorded on an X-ray film having a small gamma value designed so as to match the type of image processing to be carried out, the X-ray image is read out from the X-ray film and converted into an electric signal, and the electric signal (image signal) is image-processed and then used for reproducing the X-ray image as a visible image on a copy photograph or the like. In this manner, a visible image having good image quality with high contrast, high sharpness, high graininess or the like can be reproduced.
Also, when certain kinds of phosphors are exposed to radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays or ultraviolet rays, they store part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted by the phosphor in proportion to the amount of energy stored during exposure to the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor. As disclosed in U.S. Pat. Nos. 4,258,264, 4,276,473, 4,315,318 and 4,387,428 and Japanese Unexamined Patent Publication No. 56(1981)-11395, it has been proposed to use stimulable phosphors in radiation image recording and reproducing systems. Specifically, a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet) is first exposed to radiation which has passed through an object such as the human body in order to store a radiation image of the object thereon, and is then scanned with stimulating rays, such as a laser beam, which causes it to emit light in proportion to the amount of energy stored during exposure to the radiation. The light emitted by the stimulable phosphor sheet upon stimulation thereof is photoelectrically detected and converted into an electric image signal, and by using the image signal the radiation image of the object is reproduced as a visible image on a recording material such as a photographic film, a display device such as a cathode ray tube (CRT), or the like.
Radiation image recording and reproducing systems which use stimulable phosphor sheets are advantageous over conventional radiography using silver halide photographic materials in that images can be recorded even when the energy intensity of the radiation to which the stimulable phosphor sheet is exposed varies over a wide range. More specifically, since the amount of light emitted upon stimulation after the radiation energy is stored on the stimulable phosphor varies over a wide range and is proportional to the amount of energy stored during exposure to the radiation, it is possible to obtain an image having a desirable density regardless of the energy intensity of the radiation to which the stimulable phosphor sheet was exposed, by setting an appropriate read-out gain when detecting the emitted light and converting it into an electric signal to be used in reproducing a visible image on a recording material or a display device.
In order to ultimately obtain a reproduced image having the best image quality, a novel radiation image recording and reproducing system has been proposed in, for example, U.S. Pat. No. 4,572,060. The proposed radiation image recording and reproducing system is constituted such that a preliminary read out is carried out by exposing a stimulable phosphor sheet, on which a radiation image has been stored, to a light beam having a comparatively low level of energy, thereby to release part of the energy stored during exposure to radiation. Based on a preliminary read-out image signal obtained by the preliminary read out, information such as the intensity or the dynamic range of the radiation which was irradiated onto the stimulable phosphor sheet is ascertained. Thereafter, the final read out is carried out for reading out the radiation image from the stimulable phosphor sheet and obtaining the image signal, which is to be used for reproducing a visible image, by exposing the stimulable phosphor sheet to a light beam having a higher level of energy than the level of energy of the light beam used in the preliminary read out. Based on the ascertained information, a read-out condition for the final read out and/or an image processing condition for image processing of the image signal obtained by the final read out is adjusted automatically.
The term "read-out condition" as used herein means generically various conditions during image read out which affect the sensitivity and the gradation with which a reproduced image is obtained. For example, the term "read-out condition" means a read-out gain, a scale factor, or the power of a light beam used for image read out. Also, the term "image processing condition" as used herein means generically various conditions in image processing, which affect the sensitivity and the gradation with which a reproduced image is obtained. For example, the term "image processing condition" means the condition of scale conversion of an image signal.
The term "level of energy of a light beam" as used herein means the level of energy of the light beam to which the stimulable phosphor sheet is exposed per unit area. In cases where the energy of light emitted by the stimulable phosphor sheet depends on the wavelength of the light beam, i.e. has a distribution of sensitivity to the wavelength, the term "energy level of a light beam" means the weighted energy level which is calculated by weighting the level of energy of the light beam, to which the stimulable phosphor sheet is exposed per unit area, with the sensitivity to the wavelength. In order to change the level of the light beam, light beams of different wavelengths may be used, the intensity of a light beam produced by a laser beam source or the like may be changed, or the intensity of a light beam may be changed by moving a ND filter or the like into and out of the optical path of the light beam. Alternatively, the density of scanning may be changed by changing the diameter of a light beam, or the speed of scanning with a light beam may be changed.
There has also been proposed a system wherein , preliminary read out is not carried out, the image signal obtained by the image read out which corresponds to the aforesaid final read out is analyzed, and an image processing condition to be used in image processing of the image signal is adjusted appropriately on the basis of the results of analysis of the image signal. The method of adjusting an image processing condition appropriately based on said image signal is applicable to the cases where an image signal is obtained from a radiation image recorded on a recording medium such as a conventional X-ray film, as well as to the system using the stimulable phosphor sheet.
In the aforesaid systems, a read-out condition and/or an image processing condition is adjusted so that each reproduced image has the best quality. Therefore, the aforesaid systems are suitable for viewing each reproduced image. However, in cases where images are obtained by using different read-out conditions and/or different image processing conditions, a change in density between, for example, an image of an object which was recorded in the past and stored and an image which represents the current state of the same object cannot be ascertained accurately when the two images are compared with each other in order to investigate a change in density therebetween.
An example of the aforesaid problem will hereinbelow be described with reference to FIGS. 7A, 7B, 8A and 8B.
FIG. 7A is a schematic view showing a radiation image 6 of part (in this case, the frontal chest) of the human body, which image has been reproduced on a photographic film 5. A normal portion 6b in the radiation image 6 has an approximately uniform level of density, whereas a portion 6a affected by a disease has a level of density lower than the density at the normal portion 6b.
FIG. 7B is a graph showing a change in the abnormal density level at the portion 6a affected by the disease, which change was investigated by recording and reproducing a plurality of radiation images of the object shown in FIG. 7A with the passage of time. The disease is being cured with the passage of time. The curve A indicates ideal levels of density at which the portion 6a affected by the disease is to be reproduced when the disease is being cured. However, in cases where image read out and/or image processing is carried out so that each of the reproduced images has best quality, abnormal levels of density at the portion 6a affected by the disease among ultimately reproduced images are often represented by the curve B because of a change in the density at the portion 6a affected by the disease and other disturbing factors. In this case, the degree of the disease is judged based on the difference between the density at the portion 6a affected by the disease and the density at the normal portion 6b. Specifically, from an image which was recorded at the time T1, a density difference .DELTA.D1 between the normal level of density and the level of density on the curve A should be detected. The density difference .DELTA.D1 is large and indicates that the portion 6a has been very affected by the disease. However, there is the risk of a density difference .DELTA.D1' between the normal level of density and the level of density on the curve B being detected. The density difference .DELTA.D1' is small and indicates that the disease has been cured substantially. Also, from an image which was recorded at the time T2, a density difference .DELTA.D2 (.DELTA.D2&lt;.DELTA.D1) between the normal level of density and the level of density on the curve B should be detected. However, there is the risk of a density difference .DELTA.D2' (.DELTA.D2'&gt;.DELTA.D1') between the normal level of density and the level of density on the curve B being detected. At the time T2, the disease has actually been cured to some extent as compared with the time T1, i.e. .DELTA.D2&lt;.DELTA.D1. However, there is a risk of judging that the disease became more serious as compared with the time T1, i.e. .DELTA.D2'&gt;.DELTA.D1'.
FIG. 8A is a schematic view showing a radiation image 6' of part (in this case, sides of the vertebrae) of the human body, which image has been reproduced on a photographic film 5'. In cases where a disease which causes the density level at bone portions 6a' in the radiation image 6' to increase is diagnosed, judgment must be made based on the density at the bone portions 6a'. This is because, unlike the case shown in FIG. 7A, the bone portions 6a' and a portion 6b' outside of the bones are different organs, and a difference in density therebetween cannot be utilized for judgment.
However, when a plurality of reproduced images as shown in FIG. 8A are obtained, a read-out condition and/or an image processing condition is generally adjusted so that the density at the bone portions 6a' is constant among the reproduced images.
FIG. 8B is a graph showing a change in abnormal density level at the bone portion 6a' which change was investigated by recording and reproducing a plurality of radiation images of the object shown in FIG. 8A with the passage of time. As in the case shown in FIG. 7B, the disease is being cured with the passage of time. The curve A' indicates ideal levels of density at which the bone portions 6a' are to be reproduced when the disease is being cured. However, in cases where image read out and/or image processing is carried out so that the levels of density at the bone portions 6a' are identical among the reproduced images, the density at the bone portions 6a' is reproduced along the curve B' at approximately the same levels as the normal level of density, except for effects of disturbing factors. In this case, abnormality in density at the bone portions 6a' cannot be detected.